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Marine Ecology Progress Series

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MEPS 444:69-83 (2012)  -  DOI:

Aggregative feeding behavior in sea urchins leads to destructive grazing in a Nova Scotian kelp bed

C. Feehan1,*, R. E. Scheibling1, J. S. Lauzon-Guay2

1Biology Department, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
2Fisheries and Oceans Canada, Institut Maurice-Lamontagne, Mont-Joli, Quebec G5H 3Z4, Canada

ABSTRACT: Grazing aggregations of the sea urchin Strongylocentrotus droebachiensis drive the transition between alternative ecosystem states in Nova Scotia, from productive kelp beds to less productive barrens. This transition can be initiated by the formation of gaps within a kelp bed, containing dense aggregations of sea urchins. We examined the importance of local density of sea urchins and pre-existing gaps in a kelp canopy in mediating the formation of destructive grazing aggregations of sea urchins in a kelp bed. We transplanted 14000 adult sea urchins from a barrens on the Atlantic coast of Nova Scotia into ~4.5 m2 plots within a nearby kelp bed, at ­densities above and below a predicted threshold value for destructive grazing, and simulated disturbance to the kelp bed by removing the kelp canopy in half of the plots. Sea-urchin abundance and gap formation and expansion (as loss of kelp canopy cover) were monitored in and around plots weekly for 9 wk. Grazer-mediated gap formation began 3 wk after sea urchins were introduced, and increased for the remainder of the experiment. Our results indicate a direct linear ­relationship between sea-urchin abundance and increase in gap area within undisturbed treatments. Gaps expanded in the kelp bed at sea-urchin densities below the putative threshold for destructive grazing, indicating that the kelp bed was less resilient to grazing than predicted. Our findings provide insights into mechanisms controlling the stability of the kelp-bed ecosystem state and mediating shifts from kelp beds to barrens in Nova Scotia.

KEY WORDS: Alternative states · Destructive grazing · Aggregation · Strongylocentrotus droebachiensis · Kelp beds

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Cite this article as: Feehan C, Scheibling RE, Lauzon-Guay JS (2012) Aggregative feeding behavior in sea urchins leads to destructive grazing in a Nova Scotian kelp bed. Mar Ecol Prog Ser 444:69-83.

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